The present invention relates to internal and external sealing of fluid handling components and, more particularly, to sealing means that allow venting of internal pressure while sealing out external pressure when ambient pressure may exceed internal pressure.
In the offshore petroleum industry, petroleum reservoirs are often in deep waters where surface production platforms are not feasible. In such deep waters wellhead and christmas tree equipment are typically installed at or near the sea bed. Production from such deep wells requires the use of submarine pipelines or production risers to transport the petroleum to the surface. The ambient hydrostatic pressure is very high at great depths, often as much as several thousand pounds per square inch. Therefore, in such deep water applications, subsea equipment must be designed to seal out high pressure ambient seawater while containing petroleum products internally.
With regard to the above-mentioned problems that occur during well production, known sealing devices for use with wellhead and christmas tree connections include conventional metallic ring gaskets (10), as shown in FIG. 1. Such conventional ring gaskets are effective in sealing internal fluids at extremely high pressures. They may not, however, be effective at sealing out external water pressure when the ambient pressure exceeds the internal pressure. This is because typical ring gaskets of this type for use in subsea equipment are pressure energized seals in which sealing tightness is dependent upon internal pressure. Such seals are designed to more tightly seal as internal pressure increases relative to external pressure. As described above, situations may exist where the ambient pressure may exceed the internal pressure such that the resultant pressurization is reversed. In the aforementioned decreased or reversed pressure situations, it is likely that the pressure energized seals will become de-energized or will be insufficiently energized such that leaking, and thus damage to the well assembly, may occur.
FIGS. 1 and 2 illustrate a prior art sealing assembly (12) for a typical bolted flange connection (14) on a subsea gate valve. The sealing assembly (12) includes a metallic ring gasket (10). The gasket (10) is a pressure energized seal such that as internal pressure increases, the force that presses the gasket against the sealing surfaces (16, 18) also increases. If the gasket (10) is subjected to external pressure, the force of the external pressure acting on the outer diameter surface (20) of the gasket (10) pushes the gasket (10) away from the sealing surfaces (16,18) allowing external fluids to leak past the seal and into interior of the connection (14).
Another prior art metallic ring gasket known as an "API BX" type ring gasket (22) is shown in FIG. 3. The BX gasket (22), widely used with wellhead and christmas tree connections, is pressure energized to some extent. When subjected to high external pressures that are typical in deep water subsea wells, the BX type gasket (22) is prone to leakage due to the force of the external pressure acting on the outer diameter surface (20) of the BX gasket (22) and pushing the gasket (22) away from the sealing contact surfaces (26,28). In this instance, the pressurized fluid will leak past the seal and into the interior of the connection.
One known solution to prevent leakage into such connections as shown in FIG. 3 is to provide a secondary barrier seat (30) to prevent external pressure from reaching a first seal (22). A conventional o-ring seal is suitable for such sealing. Typically, an elastomeric material is used rather than a metal seal because it is inexpensive and it is adequate for sealing seawater.
It is common practice to conduct high pressure internal tests on these types of connections at pressures 1.5-2 times the normal internal pressure rating. Most simple elastomeric rings seal equally well in both directions. Thus, if the primary metallic ring gasket (22) were to leak during the internal pressure testing, the test fluid would enter the region (32) between the ring gasket (22) and the secondary barrier seal (30). One of both of the following two results could occur.
First, the pressurized test fluid could exert upon the connection components a separating force and result in damage to the connection.
Second, if the pressurized test fluid were to leak past the primary gasket (22) and then be contained by the secondary seal (30), such leakage may go undetected. Without any external indication that the primary seal (22) was not functioning properly, the faulty connection may be inadvertently placed into service. Once in service, the faulty connection could allow corrosive petroleum fluid to leak past the primary seal (22) and eventually corrode the secondary seal (30) causing it to fail. Retrieval and replacement of such equipment in deep water is usually difficult, expensive, and time consuming. It is therefore essential to detect and remedy seal leakage prior to shipment or installation of the equipment.